Sorry scrap that, was on Beta 7, on latest nightly this is confirmed. I've found a whole bunch of other problems with variable text antialiasing on bug 632423 - various (bad) changes occurred back on 15-16th July 2Pre builds for some pages, 19th-20th October Beta 8Pre builds for https bars, and 10th-11th November Beta
8Pre for some drop down menus where flash is enabled, these all regressed to greyscale anti-aliasing between Dec 31st-9th Jan 9Pre builds.

Should add, my cases are with hardware acceleration disabled though there may be a connection. Just to confuse things, it seems that urls in the location bar are also using greyscale AA with hardware acceleration on the latest nightly but not with hardware acceleration off.

(In reply to comment #4)
>Just to confuse things, it seems that urls in the location bar
> are also using greyscale AA with hardware acceleration on the latest nightly
> but not with hardware acceleration off.
Sorry, part 2, seems that was just down to me messing with drivers (that was with an old one), urls with hw on are fine. After some testing it seems the tooltips have been greyscale with acceleration on ever since the hardware acceleration enable box was added in Beta 5 so it was broken from the start.